At present, tinplated steel sheet, namely tinplate and aluminum sheet is widely used as a material for DI cans for carbonated beverages and beer. These DI cans are produced by the following process: cutting to a circular blank .fwdarw. drawing .fwdarw. redrawing .fwdarw. ironing several times .fwdarw. washing the coolant oil used for forming .fwdarw. surface treatment of the formed can by phosphate or zirconium salt .fwdarw. rinsing with water .fwdarw. drying .fwdarw. spray coating of lacquer on the inside of the formed can .fwdarw. color printing the outside of the formed can.
The production cost of the DI can is expensive because the production process of the DI can is complex as described above.
Recently, a resin film laminated metal sheet has been investigated to be used for a drawn and redrawn can (DRD can) and DI can with a view to eliminating the coating of lacquer on the inside of those cans, which coating takes a fair part of the can cost.
For example, a metal sheet covered with polyester resin film (U.S. Pat. No. 4,517,255) has been employed to be formed into a DRD can.
The forming process for the DRD can consists of two step drawing: drawing and redrawing. The drawing process does not involve the reduction of the can wall thickness, so the DRD process is used for forming a rather short height can.
On the other hand, the forming process for a DI can consists of two forming processes: drawing and ironing, as mentioned earlier. Under the drawing process, the metal sheet is formed into a shallow cup. This process is fundamentally the same drawing process as the DRD process. Under the following ironing process, the drawn cup is redrawn at first, then the thickness of the redrawn cup wall is successively reduced into about a third of the thickness of the original metal sheet by a several step ironing. (Redrawing and ironing are jointly performed.)
As mentioned above, since the DI process involves the reduction of the can wall thickness in order to form a thin wall and tall height can, more formability of material, higher ductility of laminated polyester resin film in particular, excellent adhesion of the polyester resin film to the metal sheet and excellent lubricating capacity of the surface to become the outside of a can are jointly required for the material used for a DI can compared to that for a DRD can.
The metal sheet covered with polyester resin film shown in U.S. Pat. No. 4,517,255 can be formed into a DRD can having excellent adhesion of the resin film to the metal sheet without any film damage, but can not be continuously formed into a DI can because of lacking in the lubricating capacity of the side of the metal sheet to become the outside of a can, and in addition, many cracks are generated in the film of the formed DI can.
For another example, polyester resin film laminated steel sheet (Laid-Open Japanese Patent Application No. SHO. 60-168643) has been employed for a DI can.
The characteristics of DI cans produced from a polyethylene terephthalate film laminated steel sheet shown in Laid-Open Japanese Patent Application No. Sho. 60-168643 deteriorate remarkably by reheating upon curing the color printing ink applied on the outside of the formed DI can. Namely, much filiform corrosion arises from the edge of DI cans reheated for curing the color printing ink during long storage in an atmosphere having high humidity and high temperature.
It is assumed that the cause of filiform corrosion is due to the deterioration of the adhesion of polyethylene terephthalate film to the steel sheet by recrystallization of polyethylene terephthalate film during reheating at above 160.degree. C., although the structure of polyethylene terephthalate film may change to the monoaxial oriented state from the amorphous non-oriented state by ironing.
Accordingly, it is the first objective of the present invention to provide a copolyester resin film laminated steel sheet or strip as a material for DI cans having excellent adhesion of the copolyester resin film to the steel sheet after forming into DI cans, excellent corrosion resistance which can be used without an inner lacquer coating after forming of DI cans and excellent external appearance after forming into DI cans.
It is the second objective of the present invention to provide a production method for a material for DI cans having excellent filiform corrosion resistance in the formed part after reheating at the temperature of 160.degree. to 200.degree. C..